With the increased use of plain paper copiers, development powders have enjoyed an advancing popularity over liquid toners. Along with the advancing use of development powders, magnetic brush units are becoming increasingly popular as opposed to cascading methods. Development powders used with magnetic brush units usually have an iron powder which serves as the carrier material. Inexpensive, untreated iron powders cannot be used in magnetic brush systems since such iron does not have sufficient stability toward rusting and has color and triboelectric charging properties adversely effected by variable humidity conditions. In order to solve the problem, those in the art have resorted to chemical plating and coating of the iron particles with polymers, oils, waxes and the like and various treatments. For example, U.S. pat. No. 3,922,381 discloses a method for applying a perfluoro acid to the surface of an electrophotographic carrier particle. Another method of treating iron particles is disclosed in U.S. Pat. Nos. 3,632,512 and 3,718,594, where the carrier particles are exposed to an aqueous acid solution, washed, then coated with a continuous film. Belgian Pat. No. 746,109 also discloses a process fo treating carrier method with an acid-wash.
Although many treatments are effective to reduce the tendency to rust, such treatments have not been completely successful in enhancing the triboelectric properties of the carrier particles. Untreated iron carrier particles when mixed with a standard toner are known to have a charge to mass ratio (C/M) of approximately 7 .times. 10.sup.-6 coul/gm. It has been found that the perfluoro acid treatment of the iron carrier particles disclosed in U.S. Pat. No. 3,922,381 has increased the C/M to a range of 13 .times. 10.sup.-6 to 17 .times. 10.sup.-6 coul/gm. It would be desirable to be able to increase the C/M further as an increasing C/M leads to increasingly sharper contrast and lower background in electrophotographic copies.